1. Field of the Invention
The present invention relates to methods and apparatus for comminuting a process mixture and, more particularly, to methods and apparatus for comminuting a process mixture in an agitated media.
2. Description of the Prior Art
Various methods and apparatus have been used to grind particulate solids. These include ball mills, pebble mills, roll mills, send mills, and tube mills. Considered to be generally illustrative of this art are U.S. Pat. Nos. 1,577,052; 1,956,293; 2,903,191; 3,131,875; 3,298,618; 3,329,348; 3,337,140; 3,339,896; 3,432,109; 3,591,349; and 3,628,956; and British Pat. No. 1,038,153.
Distinguished from the above apparatus and methods used for grinding are comminuting devices and methods. As used herein, comminuting is specifically defined to include dispersion and deflocculation processes in addition to grinding processes. Attendant advantages of the comminuting type mill are the capacity for simultaneously mixing two or more different solids, or for suspending one or more particulate solids in a liquid.
Prior art comminuting apparatus and methods which have been found to be particularly effective have included agitated-media comminuting apparatus. Such comminuting means generally include a vessel that contains a bed of comminuting elements that are agitated by members connected to a rotating shaft. Typically, the solid particles are ground to a particle size in the range of 50 to 0.5 microns and, in certain instances, the particles are ground even smaller. A substantial advantage of the agitated media type comminuting mills, as compared to vibratory mills for example, is that comminution occurs primarily between the comminuting elements of the agitated media and does not involve the vessel walls. Consequently, mechanical wear of the agitator shaft and the inner wall of the vessel is considerably reduced. Another advantage of agitated-media type comminuting mills is that the comminuting vessel remains stationary so that these mills are less cumbersome than vibratory type mills. Examples of agitated-media mills are discussed in U.S. Pat. Nos. 2,764,359; 3,008,657, 3,075,710; 3,149,789; and 3,539,117; British Pat. Nos. 716,316, 1,331,662; and German Patentschrift Nos. 1,214,516; and 1,233,237.
Generally, agitated-media comminuting mills are described as batch-type, circulation-type, or continuous-type mills. In a batch-type mill such as described in U.S. Pat. No. 2,764,359, a selected quantity of a process mixture is placed in a vessel together with the comminuting media and the comminuting media is then agitated by an agitator. The properties of the process mixture are such that, as a general rule, the mixture behaves as a fluid. Generally the process mixture includes a liquid that tends to hold the solid particles in suspension as they are being comminuted with the solids in the suspension comprising 20-50% by volume and 40-65% by weight. Alternatively however, the process mixture may be substantially comprised of one or more kinds of solid particles that are to be comminuted. In the case of comminuting particulate solids where no liquid is present, the agitated-media mill generally includes two agitators. These agitators have substantially parallel shafts that are laterally disposed. The comminuting media is agitated until the mixture is ground, dispersed and/or deflocculated as required. When the mixture is sufficiently comminuted, the batch type comminuting mill is stopped and the processed comminuted material is removed.
The continuous-type agitated-media comminuting mill such as described in U.S. Pat. No. 3,149,789 is somewhat similar to the batch-type mill except that the mill is more elongated and the process mixture is steadily introduced to the comminuting vessel at one end and the comminuted mixture is removed at the opposite end. Usually, the mixture is introduced at the bottom of the vessel and removed from the top of the vessel with the verticle progression of the mixture through the vessel being horizontally stratified such that the particle density and size distribution of the mixture remain substantially the same at each level of the vessel.
In circulation-type agitated-media comminuting mills such as described in U.S. Pat. No. 3,998,938, the comminuting vessel is somewhat similar to that of the batch-type mill but the process mixture is repeatedly recirculated at a high flow rate through the comminuting vessel as the mixture is being comminuted. The high flow rate, sometimes referred to as "streaming speed", together with the recirculation of mixture results in an unexpectedly rapid comminution of the mixture and provides a product with a narrow particulate size distribution. The streaming speed is generally measured in terms of the volume of the agitated-media comminuting mill which is defined as the difference between the volume of the comminuting vessel and the volume displaced by the agitator together with the comminuting media. Typically the volume of the agitated-media mill is in the range of about 35 to 50 percent of the total volume of the comminuting vessel. Also typically the streaming speed is at least about 30 and preferably between 50 and 300 volumes per hour.
It is believed that the improved comminuting performance of the batch, continuous, and circulation agitated-media mills is due to the action of the agitated media and, in particular, to the collisions between the comminuting elements. These collisions impinge on the solid particles of the process mixture causing the particles to be physically divided and sub-divided. Furthermore, it appears that, in certain instances, changes in the properties of the particle solids also occurs. This action of the agitated media has been variously described in connection with the kinetic energy of the comminuting elements and the mean free path between collisions of the comminuting elements. Indeed the comminuting elements used in agitated-media comminuting mills are generally smaller than those employed in other types of mills in order to increase the rate of collisions between comminuting elements and, thereby, increase the rate of comminution.
In circulation comminuting mills where vertical flow of the process mixture is substantial, the additional factor of streaming speed appears to have a further effect on the rate of comminution. The circulation-type mills comminute the particles in the process mixture at a rate that is unexpectedly higher than the rate of processing that is obtained with considerably larger batch-type agitated-media mills. The higher the streaming speed, the higher the frequency that any given volume of the process mixture passes through the agitated-media mill and the faster the mixture is comminuted. In the circulation-type comminuting mills, an additional factor in the improved comminuting results is believed to be a dynamic screening effect in which the agitated media acts as an effective screen that tends to retain the larger, less comminuted particles in the comminuting mill until they are comminuted by the agitated media. Accordingly, a circulation-type agitated-media mill has a higher processing capacity than batch-type and continuous-type mills of comparable size and capacity.
As another result that is apparently due to the dynamic screening action of the agitated media in the circulation-type comminuting mill, it has also been found that additional, ingredients can be intermittently or continuously added to the process mixture during the operation of the circulation-type agitated-media comminuting mill to disperse the additional ingredients throughout the mixture such that the comminuted product includes ingredients and/or proportions having controlled particle sizes and/or chemical or physical properties.
In many applications, agitated-media comminuting mills of the prior art have proven effective for certain applications for comminuting the process mixture to provide a product having small particle size of substantially uniform size distribution. Examples of process mixtures that have been thus comminuted are ferromagnetic materials, paint pigments, dye pigments, tungsten carbide, certain carbon blacks and transparent oxides. However, in agitated-media comminuting mills of the prior art, whether of the batch, circulation, or continuous type, it has now been found that, where there is substantially no random motion of the comminuting elements, there is substantially no comminution of the material to be processed. Moreover, it has also been found that where there is insufficient relative motion between the comminuting elements and the agitator, there is again substantially no comminution of the mixture. This condition does not appear to be dependent upon whether the mixture includes a suspension liquid but, rather, upon the degree of random motion of comminuting elements and upon the fluid properties of the mixture. Specifically, it is believed that the condition of insufficient random motion of the comminuting elements is related to the kinetic condition of the grinding media. Also, it is believed that the condition of insufficient motion of the agitator with respect to the agitated media is related to the tendency of the process mixture to rotate as a solid with the agitator due to the viscous properties of the mixture and the surface friction of the inner wall of the comminuting vessel. More specifically, it is believed that when these properties and conditions establish a laminar flow condition in the boundary layer of the mixture, there is an insufficient friction against the rotation of the mixture. Consequently, the mixture tends to rotate as a solid body with the agitator and there is insufficient mutual action between the comminuting elements and the agitator. As used herein, boundary layer is specifically defined to mean the layer of mixture adjacent the surface of the inner wall of the comminuting vessel.
The tendency of the mixture to rotate with the agitator as a solid mass has been found to be particularly prevalent among certain particulate materials that tend to flocculate together as a single amorphous mass. Common examples in which there is substantially no random motion of the comminuting elements or there is insufficient relative motion between the comminuting media and the agitator are found in applications where a mixture, while not necessarily having a substantial tendency to flocculate before comminution is begun, undergoes a change in its properties during comminution such that the rate of reflocculation increases to the extent that the partially comminuted mixture rotates with the agitator as a solid. For example, in grinding aluminum powder into a fine aluminum dispersion suitable for the manufacture of aluminum paint and related materials, the aluminum powder granules have been found to have a tendency to reflocculate such that they tend to rotate with the agitator as a single mass. As another example, chocolate has also been found to exhibit this tendency for reflocculation-especially under conditions of high moisture content (e.g. in the range of 3-6% by weight).
Prior to the present invention, a need existed for a method and structure for establishing random motion of the comminuting elements, and for opposing the tendency of certain process mixtures to rotate as a solid with the agitator thereby establishing relative motion between the agitator and the comminuting media to comminute the mixture to be processed. Various liners and coatings have been used in tube mills, ball mills, and other grinding mills known to the prior art. Examples are found in U.S. Pat. Nos. 3,202,364; 2,909,335; 1,741,604; 1,307,952; 2,334,256; 639,409; B441,416; and 1,986,103. However, none of these devices were suitable for use in an agitated media comminuting mill as described above such that they would be effective either to establish random motion of the comminuting elements or to prevent the solid body rotation of the mixture in order to comminute the mixture. Accordingly, there was a need for a method and apparatus for establishing random motion of the comminuting elements, and for preventing solid body rotation of the process mixture in agitated media comminuting mills.